An exhaust gas emitted from diesel engines contains fine particles (particulate matter) based on carbon such as soot and high-boiling-point hydrocarbons. When such exhaust gas is released in the atmosphere, it may adversely affect human beings and the environment. For this reason, a ceramic honeycomb filter, which may be called “honeycomb filter” in short later, has been disposed in an exhaust pipe connected to a diesel engine to purify an exhaust gas by removing particulate matter. As shown in FIG. 1, a honeycomb filter 10 comprises a ceramic honeycomb structure 11 comprising porous cell walls 2 forming a lot of flow paths 3, 4, and an outer peripheral wall 1, and plugs 6a, 6b alternately sealing both ends 8, 9 of the flow paths 3, 4 in a checkerboard pattern. An exhaust gas flows into the outlet-side-sealed flow paths 3, passes through the cell walls 2, and exits from the inlet-side-sealed flow paths 4. While the exhaust gas is passing through pores on and in the cell walls, particulate matter is captured on the cell wall surface and in the pores. A large pore size reduces pressure loss while the exhaust gas passes through the honeycomb filter, but lowers a particulate-matter-capturing ratio. On the contrary, a small pore size increases the capturing ratio as well as the pressure loss. Too small a total pore volume increases the pressure loss, while too large a total pore volume lowers the strength of the honeycomb filter. Accordingly, the pore size and the pore volume are controlled depending on applications, by adjusting the amount of a pore-forming material such as coal powder and wheat powder added to a material powder. Recently, the gas-containing, hollow resin microcapsules described in JP 2003-38919 A have come to be used in place of the coal powder and the wheat powder as a pore-forming material.
With unevenness among production lots, microcapsules in different production lots may provide sintered honeycomb structures with different pore volumes, even if they are added in the same amount to ceramic powder. The pore volume of the sintered honeycomb structure may be called simply “pore volume” below.
WO 2005/068398 discloses a method for producing a honeycomb structure stably having a constant pore volume even if microcapsules stored for a long period of time are used, by regulating the weight of a gas contained in the microcapsules stored at 40° C. for 4 weeks. However, the method described in WO 2005/068398 needs a preliminary test of storing microcapsules at 40° C. for 4 weeks, making it difficult to know a proper amount of the pore-forming material to be added in a short period of time if the production lots of microcapsules are changed. Accordingly, this method fails to cope with a rapid production change, and needs high cost in storing microcapsules.
JP 2005-314218 A discloses a method for producing a porous structure having stable pore properties comprising extrusion-molding part of an extrusion material for honeycomb structures whenever material lots are changed, measuring the pore properties of its molded article after sintering to have information about the unevenness of the extrusion material, and adjusting the amounts of a pore-forming material and water added to the material based on the information. The method described in JP 2005-314218 A, however, needs a lot of time in molding, sintering and evaluation, making it difficult to know a proper amount of a pore-forming material in a short period of time. It also suffers a high production cost.
As described above, the conventional technologies need a lot test of a pore-forming material over a long period of time. Accordingly, a technology capable of easily stabilizing the pore volume of the honeycomb structure in a short period of time has been demanded.